Treatment of silicon-iron alloys



June 20, 1944. s LBURGWW 2,351,922

TREATMENT OF SILICON-IRON ALLOYS Filed March 28, 1941 2 Sheets-Sheet lWITNESSES: INVENTOR Patented June 20, 1944 TREATMENT OF SILICON-IRONALLOYS Stephen L. Burgwin, Wilkinsburg, Pa" assignor to WestinghouseElectric & Manufacturing Company, of Pennsylvania East Pittsburgh, Pa.,a corporation Application March 28, 1941, Serial No. 385,759 8 Claims.(Cl. 148-215) This invention relates generally to magnetic material andto the heat treatment of siliconiron alloys to reduce magnetostriction.

Silicon-iron alloys containing up to 6% of silicon have been utilizedfor a number of years .as the magnetic material in diflerent electricalapparatus, and particularly as the core member of transformers. Manyprocesses and methods have been employed for heat treating thesilicon-iron alloys and fabricating them into sheet or strip formsuitable for use as the magnetic cores of electrical apparatus. Theknown processes and methods have for their objective the development ofthe magnetic characteristics of the alloys by reducing the impurities inthe alloys, relieving working strains, and/or imparting a preferredorientation to the grain structure of the alloy. The silicon-iron alloysnow commercially produced in sheet or strip form have excellent magneticcharacteristics.

In a number of commercial installations of electrical apparatus, such aslarge power transformers, which utilize the now commercially producedsilicon-iron alloys in strip form as the magnetic core member, it hasbeen found that undesirable and objectionable noise is developed whenthe transformer is energized. The noise developed in a transformer canbe traced directly to the magnetostriction of the siliconiron alloysheets or strips utilized as the core member of the transformer. Themagnetostriction of magnetic material is recognized as being theexpansion and contraction of the magnetic material when subjected to amagnetizing force.

For the purpose of this invention, thekind of magnetostriction ofparticular interest and the quantity to be measured is AL/L or thechange in length per unit length in the direction of magnetization underthe magnetizing force applied.

An object of this invention is to provide for reducing themagnetostriction of silicon-iron a1- loys which have been fabricatedinto strip form and treated to develop the magnetic characteristics ofthe alloy.

A more specific object of this invention is to subject silicon-ironalloys which have been fabricated into strip form and treated to developthe magnetic characteristics of the alloys to an annealing treatmentunder tension to effectively reduce the magnetostriction of the alloystrip.

Other objects of this invention will become apparent from-the followingdescription when taken in conjunction with the; accompanying drawings,in which: I

Figure 1 is a schematic representation of the apparatus employed inpracticing the method of this invention,

Fig. 2 is a schematic plan of test apparatus utilized in measuring themagnetostriction of alloy strips produced in accordance with thisinvention,

Fig. 3 is a view in elevation, with a part removed, of a portion of theapparatus of Fig. 2,

Fig. 4 is a graph, the curves of which illustrate the magnetostrictionvalues obtained on alloy strips by utilizing the test apparatus of Fig.2,

Fig-5 is a schematic plan of a further test apparatus utilized formeasuring the magnetostriction of assembled core members such as areemployed in transformers,

Fig. 6 is a graph, the curves of which illustrate the losses encounteredin alloy strips treated in accordance with this invention as comparedwith the losses found in similar alloy strips which have been subjectedto only a part of the treatment of this invention, and

Fig. 7 is a graph illustrating the magnetostriction as measured by theapparatus of Fig. 5 for core members of transformers which have beengiven difierent treatments.

The silicon-iron alloys containing up to 6% of silicon are well knownand the processes for developing the magnetic characteristics of suchalloys have been commercially practiced in the past. For example,silicon-iron alloys containing up to about 4 A of silicon arecommercially produced in sheet or strip form by subjecting them todifferent heat treatments, including the step of cold rolling them tothe final stages and then subjecting the resulting strip to an anneal ata temperature of 900 C. or higher to develop the magneticcharacteristics of the alloys. Silicon-iron alloys containing highercontents of silicon up to about 6% are not being commercially producedby cold rolling processes, since the higher silicon content renders themsomewhat brittle and it has therefore become customary to hot work suchalloys to the final stages after which a final anneal is employed forrelieving working strains. Other methods are well known and practicedfor developing the magnetic characteristics of the silicon-iron alloyscontaining up to 6% of silicon.

' In order to reduce the magnetostriction of the commercially producedsilicon-iron alloys containing up to 6% of silicon, in accordance withthis invention the commercially produced alloy sheets or strips aresubjected to a further heat treatment whereby the alloyed components of,the strip are in such a condition that the magapplied for short periodsof netostriction of the commercially produced alloys is effectivelyreduced. For the purposes of this invention the terms sheet and stripare synonymous in their meaning as employed in this application.

Referring to Fig. 1 of the drawings, there is schematicall illustrated arepresentative apparatus for practicing the method of this invention.The apparatus consists of an annealing furnace l having open endsthrough which the strip [2 of silicon-iron alloy which has already beentreated and fabricated to develop its magnetic characteristics is.passed from a supply reel H to a winding reel l6. A brake I8 isassociated with a supply reel H for applying a suitable dra thereto toimpart a predetermined tension to the sheet or strip l2 as it is drawnthrough the annealing furnace Hi, the purpose of which will to the stripl2 as it is subjected to the annealing temperature is below the elasticlimit of the alloy strip and preferably between 500 and 2000 pounds persquare inch. The elastic limit as employed in describing this inventionand in the claims is a point on the stress-strain curve which representsa stress equal to half of the maximum stress which the strip will hearat the heating temperature without breaking. Where tension below theelastic limit defined hereinbefore is time such as one to two minutes,it is found that no permanent elongation, except that which isconcomitant by the tensioning within the elastic limit, results. Inpractice the strip i2 is drawn through the furnace l0 at a speed whichwill p'ermitthe heating of the strip to the desired annealingtemperature while the winding reel I6 is positioned far enough away fromthe furnace 10 to permit the strip l2 to cool to a temperature of about100 C. before it is wound on the reel Hi.

It is not known exactly what change takes place in the components orstructure of the alloy when subjected to the hereinbefore describedannealing treatment under tension. The results obtained, however,illustrate quite definitely that the tension applied during the annealin air at a temperature between the Curie point of the alloys and 825 C.cooperates to effectively reduce the magnetostriction of thecommercially produced silicon-iron alloy strip.

In order to illustrate the beneficial efiects of the treatment of thisinvention, reference may 'be had to Fig. 4 of the drawings, the curvesof which illustratethe magnetostriction of siliconiron alloy sheets orstrips after different treatments. Curve 20 represents a measure ofmagnetostriction for a silicon-iron alloy strip which contains about El/2% silicon asmeasured at different flux densities in a strip havingsurfaces which are free from films. Curve 22 represents the measure ofmagnetostriction for a stripof the same material when subjected to onlythe heat treatment utilized inthe method of this invention. Curve 24represents the magnetostriction of another strip of the same materialvwhich has been subjected to the annealing treatment of this inventionunder tension.

In particular, curve 22 represents the measure of magnetostrictionobtained by subjecting the commercially produced silicon-iron alloysheet or strip to a temperature of 700 C. in air while curve 24represents the measure of the magnetostriction obtained for the samecommercially produced silicon-iron alloy strip when subjected to anannealing. temperature of 700 C. while maintained under a tension of2000 pounds per square inch. From the curves it is quite evident thatthe treatment of this invention effectively reduces the magnetostrictionof the commercially produced silicon-iron alloy strip.

Since the methods of measuring magnetostriction are not well known, andsince it has been found necessary to utilize more than on method formeasuring the magnetostriction, it is believed to be desirable tobriefly-explain the apparatus and method employed in measuring themagnetostriction as represented by the curves 20, 22 and 24 of Fig. 4 ofthe drawings.

Referring to Fig. 2 of the drawings, there is illustrated the apparatusfor measuring the magnetostriction of silicon-iron strips employing adirect current excitation means. In this apparatus the magnetic circuitconsists of four strips 26 of the silicon-iron alloy arranged withdouble lap joints in a 25 cm. test frame. The magnetostriction of onlyone of the strips 26 is measured, the other three strips serving tocomplete the magnetic circuit. The test strip 26 whose magnetostrictionis to be measured is clamped as by means of the screws 28 in anon-magnetic holder 'formed of the two bars 30 and 32, the holder beingdisposed to slide over the strip 26. The

- bar 32 carries bronze bearing balls 34, as shown in Fig. 3, upon whichthe test strip 26 rests.

In order to measure the elongation of the test strip 26, a roller member36 having a mirror 38 at one end thereof is disposed between the teststrip 26 and the bar 30 of the holder for rotating when the test strip26 is elongated. As illustrated, the mirror 38 is so positioned as todeflect a beam of light from a lamp 49 which is focused upon a splitphotox unit 42 which is disposed to be connected to a galvanometer 44 bymeans of the switch 46, so that the deflection of the galvanometer isproportional to the deflection of the light beam upon the photox unit.

In order to energize the magnetic circuit formed by the strips 26, asource of direct current, such as the battery 48, is disposed to beconnected through the double-throw switch 50 to the winding 52 disposedabout the strips forming the core member. Variable resistors 5| and 53are connected in series with each other and the battery, a switch 55being provided for shunting resistor 53. The double-throw switch 50 isemployed for controlling the direction of the energization of theassembled test specimen.

Since it is desired to measure both the elongation of the test strip 26and the flux densities under which such elongation is obtained, awinding 54 is disposed about the assembled test strips 26 and is adaptedto be connected to the galvanometer 40 by means of the double-throwswitch 46. Thus with the rotation of the mirror deflecting the lightbeam on the photox unit 42, a measure of the elongation of the strip 26can be obtained on the galvanometer 44 as well as a measure of theflux-density causing the measured elongation.

Another winding 56 is disposed about the test assneaa 4 netostrictionfor inductions up to 20,000 Gausses.

In operation with the apparatus and test sheet or strip 28 set up asshown in Figs. 2 and 3, the

double-throw switch 48 is thrown to the right to connect winding 54 tothe galvanoi'neter 44. The

double-throw switch 58 is then moved to the right to connect the battery48 to winding 52 to energize it. Resistor Si is then adjusted until theinduction as measured by the galvanometer upon reversal of switch 58 isequal to the desired value at which the test is to be conducted.

Resistor 53 is then adjusted to a value where the simultaneous reversalof the double-throw switch 58 and the opening of switch 55 so affectsdeenergization of winding 52 that the gal-' vanometer 44 indicates zeroinduction. With this adjustment of the resistors 5i and 53, when theswitch 48'is thrown to the left to connect the photox unit 42 in circuitwith the galvanometer 44, the deflection of the galvanometer whichoccurs upon the simultaneous reversal of switch 58 and the opening ofswitch 55, is a-measure of the magnetostriction of the strip 26 undertest. That this deflection is a measure of the magnetostriction is quiteapparent in that the roller 36 and mirror 38 move as the test stripcontracts as the induction is changed from the maximum desired value tozero induction by operating the switches 58 and 55. This measure ofmagnetostrictionis for a unit length of the test strip extending fromthe center line of the overlapping joint at the secured end of the teststrip to the point of contact of the roller 36.

coil, due to its inertia, will tend to remain substantially fixed,

with the pick-ups 84 disposed at the opposite ends of a leg of the coremember 88 as shown in Fig. 4, andconnected in series opposition, thevibrations for given inductions are recorded through the amplifier andthe wave analyzer. The pick-ups are disposed at a number of differentlocations at the end of the leg of the core member in order to get anaverage recording of the vibration of the core member. In recording thevibrations, the amplitude of the l-cycle fundamental'frequencyt alone ismeasured since for the purpose of this invention it is very closelyproportional to the maximum amplitude of vibration. The values recordedby the wave analyzer are thus equal to the total vibration of the coreleg and when analyzed with respect to the length of the core member,give magnetostriction values-which are quite close to the valuesobtained on individual test strips when the apparatus of Fig. 2 isemployed for measuring the magnetostriction. In measuring thevibrations, the amplifier and wave analyzer are first calibrated bymeasuring a known vibration so that the correct value of the vibrationof the core member at any given induction can be obtained.

Referring to Fig. 71 of the drawings, there 'is illustrated thediflerence in the magnetostriction obtained with two core members of thesame size constructed from the same commercially produced silicon-ironstrips which have been subjected todifferent treatments. Curve I00represents the magnetostriction measured on a core member of thecommercially produced strips which have been subjected to only thefurther anneal at a temperature oi 100 C., while curve I02 representsthe magnetostriction measured at diflerent flux densities fora coremember of identical size formed of identical silicon-iron strip materialbut which has beensubjected to In order to illustrate that the method ofthis 4 Referring to Fig. 5 of the drawings, there is 4 shown theapparatus utilized for measuring the magnetostriction of a core member88. In this apparatus the core member 88 is built up on a support 98 ofnon-magnetic steel and is clamped in position by means of a top clampingframe (not shown) secured to the support 88. A winding '82 is providedabout'the core member 88 and disposed to be connected to a 60 -cyclesource of power. Matched vibration pick-ups 94 are positioned at eachend of one of the legs of the core member 88 and are electricallyconnected in series to an amplifier 88 which, in turn, is elec tricallyconnected to a wave analyzer-88. The core member is then energized atdifferent flux densities at 60-cycle excitation and the vibrations ofthe leg of the core member measured to obtain the magnetostrictioneffect of the core memher.

The vibration pick-ups 84 are well known and need not be described orshown in'detail, the pick-ups consisting generally of a small coilfloating in a magnetic field produced by a permanent magnet firmlyattached to the case of the casing of the pick-up will vibrate while thean annealing treatment in air at a temperature of 780 C. while under 'atension of 1200 pounds per square inch, and thereafter cooled undertension to approximately C. The curve I88 and I82 clearly illustratethat the method of 7 this invention when applied to commerciallyproduced silicon-iron alloys which have been fabricated into strip formand treated to develop their magnetic characteristics is effective forreducing the magnetostriction of core members formed from the treatedalloy strip.

The method of this invention is also effective in reducing the losses incore members constructed from the commercially produced alloy sheets orstrips as illustrated by Fig- 6 of the drawings. In this figure, curveI84 represents the true watts loss obtained 'at difierent flux densitiesfor the core member upon which the magnetostriction curve I88 of Fig. '7is obtained while curve I88 represents the true wattsloss at diflerentflux densities for the core member treated inaccordance with thisinvention and upon which curve I02 01' Fig. 7 is based.

While it is not known exactly what happens in the alloy when it istreated in accordance with the method of this invention, it is quiteapparent that the combination of annealing in air at a sets thecomponents of the alloy as to efiectively reduce the magnetostrictionand insure the maintenance of the reduction of themagnetocharacteristiol.

magnetic Although'this invention has been described with reference to aparticular embodiment thereof, it is, of course,"not to be limitedthereto except by the scope of the appended claims.

1 I claim asmyinvention:

actress strlction at thg operating temperatures of the MAL treated alloystrip through an chamber, subjecting the alloy strip to a temperaturebetween 700 C. and 825 C. in the annealing 1. In the method of reducingmagnetostriction in a silicon-iron alloy containing up to 6% of siliconand which has been fabricated into strip form and treated to developmagnetic characteristics, the steps of, subjecting the treated.

alloy strip to an annealing treatment at a temperature between the Curiepoint of the alloy and 825 C. while applying tension to the strip, thetension applied to the strip during the anneal being below theelasticlimit of the alloy strip.

the tension being the only treatment applied to the alloystrip inaddition to said annealing treatment for effectively, reducing themagnetostriction of the alloy strip.

2. In the method of reducing magnetostricof silicon and which has beenfabricated into strip form and treated to develop its magneticcharacteristics, the steps of subjecting the treated alloy strip to anannealing treatment at a temperature between 100" C. and 825 C. in air,while applying tension to the strip, the ten- .sion applied to the stripduring the air anneal being below the elastic limit of thealloy strip,

'the tension being the only treatment applied to the .alloy stripinaddition to said annealing treatmentfor effectively reducing themagnetostriction of the alloy strip.

H 3. In the method of reducing magnetostriction in a silicon-iron alloycontaining up to-8% strip form and treated-to develop its magneticcharacteristics, the steps of subjecting the treated alloystrip to anannealingtreatment at a temperature between I00 0. and 825 C. whileapplying a tensionof between 500 and 2000 pounds persquare inch to thestrip, the tension being the only treatment applied to the all y stripin addition to the annealingtreatment, the tension applied cooperatingwith the anneal to eilbctively reduce the magnetostriction of the alloystrip.

4.'In the method of reducing magnetostriction in a silicon-iron alloycontaining up to 0% of silicon and which has been fabricated into stripform and to develop its magnetic characteristics, the steps of.subjecting the treated alloy strip to anannealing treatment at atemperature between tension being the only treatment applied to thealloy strip in addition to the annealing treatment, the-tensionappliedcooper ting with the 700' C. and- 825C. inv air, while applying atensionof between 500 and 2000 poundsper square inch to the strip, the

chamber while applying tension to the strip as it passes through theannealing chamber, the tension applied being below the elastic limit ofthe alloy strip, and cooling the heated alloy strip under tension'fromthe annealing temperature to a temperature of about 100' 0., the Ytension being the only treatment applied to the alloy strip during theheating and cooling of the strip, the. tension applied cooperating withthe heating and cooling treatment to effectively reduce themagnetostriction of the alloy strip.

6. In the method of reducing magnetostriction in a silicon-iron alloycontaining up to 6% of silicon and which has been fabricated into stripform and treated. to develop the magnetic tion in a silicon-iron alloycontaining up to 6% characteristics, the steps of passing the heattreated alloy strip through an annealing chamber having, open' ends,subjecting the alloy strip to a temperature between 100 C. and 825 'C.in the annealing chamber in air while applying tension to the strip, thetension applied being below the elastic limit ofthe alloy strip, andcooling the heated alloy strip in air under tension from the annealingtemperature to a temperature of about 100 0., the tension being the onlytreatment applied to the alloy strip during the heating and cooling ofthe strip, the tension applied cooperating with'the air' heating andcooling treatment to-eifectively reducethe m8!- netostriction of thealloy strip.

'I. In the method of reducing magnetostri tion in a' silicon-iron alloycontaining up to 6% of silicon and which has been fabricated into ofsilicon and which has been fabricated into strip form and treated todevelop the magnetic characteristics, theg steps of passing the' heattreated alloy strip through an annealing chamber, subjecting the alloystrip to a temperaturebetween 700 C. and 825 C. in the annealing chamberwhile'applying a tension of between ,500 and 2000 pounds per square inchto the strip.

and cooling the heated alloy strip under tension from the annealingtemperature, the tension being the only treatment applied to the alloyanneal to effectively reduce the magnetostricticn 10! the alloystrip. a0. In the methodof reducingmagnetostriction in a silicon-iron alloycontaining up to 6%,

of silicon and which has been fabricated into strip form and todevelopthe magnetic the-steps ofpassing the heat ing'withthe-anneal'to-eflectively magnetostrlction of the alloy strip.

strip during the heating and cooling of the strip,

the tension applied cooperating with the heating and cooling treatmentto effectively reduce the magnetostriction of the alloy strip.

8. In the method ofreducing magnetostrlc tween 100 C. and 825C. in theannealing chamber in air-while applying a tension of between 500 and2000 pounds per square inch'to the strip for a period of time of fromone totwo minutes, the tension being'the only treatment applied tothealloy strip in" addition to the an-.

nealing treatment, thetension applied cooperatne t

